i have a problem which is i have no h/w or c/h after a while the fan neon flashes up red i go to the fault section and it suggests the air pressure switch needs replacing can anyone confirm if this is correct

Does the fan run?? If so it may well be the aps, or it might be a blocked venturi, kinked pipes etc. If the fan doesn't run then its the pcb or fan itself. I would suggest at that point you call an rgi

Powell30 is giving you all the correct information. It probably is aps stuck in normally open by sounds of it. However if your not competent you really shouldn't be undertaking work (as defined by gas safety regs) on your appliance.
Get an RGI in.

Indeed. It never fails to amaze me when I read what people do, how much time and money they spend on changing the wrong parts, and often the problem is solved by a pro for under £100 including a service.